Travelling wave linear accelerators have been disclosed in the past wherein feedback of the remnant RF power from the output of the linear accelerator is combined in suitable phase relationship with input power from the RF source using an RF bridge. With proper phase conditions, the RF power entering the accelerator can be increased above that available from the source by a factor which depends upon the total attenuation in the feedback loop and upon the RF bridge ratio. Such systems are disclosed by R. B. R. Shersby-Harvie and L. B. Mullett in "A Travelling Wave Linear Accelerator With R.F. Power Feedback, and An Observation of R.F. Absorption by Gas in Presence of a Magnetic Field," Proceedings of the Physical Society, pages 270-271, Feb. 3, 1949, and P. M. Lapostolle and A. L. Septier, "Linear Accelerators," North-Holland Publishing Company, Amsterdam, pages 56-60 (1970).
A variety of RF bridge circuits, suitable for this feedback application, include coaxial and waveguide hybrid junctions, short branch couplers, coaxial and waveguide hybrid rings, etc., each of which can be represented as an eight terminal network arranged so that the following specific conditions are satisfied. Assuming four transmission lines connected to an RF bridge, as shown in FIG. 1, (a) arms 1 and 3 should be independently matched to the bridge when arms 2 and 4 are terminated by their characteristic impedances; (b) a high degree of isolation should exist between arms 1 and 2 so that power fed into either arm 1 or arm 3 is transmitted to loads in arms 2 and 4 only; (c) conversely, arms 2 and 4 should be balanced with respect to each other so that RF power entering either arm is delivered to loads at arms 1 and 3 only; and (d) there should be no power circulating within the bridge.
Typical prior application of the RF feedback principle is shown schematically in FIG. 2. Power, P.sub.1, from an external source is combined with correctly phased remnant power, P.sub.3, from the accelerator so that, after an initial transient build-up period, a steady state power level of P.sub.2 =(1+n)P.sub.1 appears at the input to the accelerator. The bridge ratio, n, is defined as the ratio of RF powers that the bridge is designed to combine. When source power is applied to the system, it is shared initially between the accelerator and load arms; and after one recirculation through the accelerator, for a unity ratio bridge and a 3 dB loss in in the feedback loop, and with correct phasing, the accelerator input power will increase to 112.5 percent of the source power, while power to the load will be reduced to 12.5 percent of the source power. The accelerator input power will continue to build up with each recirculation; and after five traversals of the feedback loop, the input power will be 194 percent of the source power (97 percent of the steady state value). The time for each RF transit is determined primarily by the group velocity of the accelerator structure. At completion of the build-up process, the accelerator input power level will be double that of the source, while the power in the load arm will be reduced to zero, as shown in FIG. 3. The mutually conjugate properties of the bridge arms 1 and 3 ensure that even during the RF build-up process, a constant impedance is presented to the external RF source.
With RF recirculation techniques, the stability of the accelerator input power depends critically on maintaining a specific phase relationship between the feedback power and the source power, i.e., the overall electrical length of the feedback loop must be maintained at a constant "resonant" value. Thus, phase changes in the feedback loop, caused either by changes in temperature of the accelerator structure or by departures from the correct operating frequency, result in a loss of input power to the accelerator and, therefore, a change in beam performance.
In prior accelerator feedback applications, the power level at the input to the accelerator was monitored so that the loss of RF buildup, due to a change in phase of the feedback loop, could be detected and corrected by adjustment of a high power RF phase shifter located in the accelerator feedback arm, as shown in FIG. 2. It should be noted that, although adjustment of this external phase shifter can maintain the total electrical length of the feedback loop at the correct resonant value, by compensating for temperature or frequency related phase changes of the accelerator waveguide, the phase slip error between the electron beam and the accelerating RF field within the wavequide remains uncorrected.
Travelling wave resonant ring accelerators were successfully demonstrated on a commercial basis during the early 1950's when the first linear accelerator systems developed specifically for megavoltage radiotherapy were placed into clinical service. These early accelerators were RF energized by a 2 MW wartime developed radar magnetron; and to greatly simplify patient setup procedures and ensure accuracy, the accelerators were isocentrically mounted (Howard-Flanders, P., and Newberry, G. R., 1950, Brit. J. Radiol., 23, 355). Since rotation of the accelerator around the patient was one of the tri-axii conditions of the isocentric mounting and because the magnetron frequency stability was known to be marginal, the early radiotherapy accelerators proved to be excellent candidates for RF feedback because the frequency sensitivity and the length of the accelerator waveguide could be reduced to provide a more compact and maneuverable system, while still achieving the desired 4 MeV loaded beam energy. With the advent of the tunable magnetron, with use of AFC controls and beam bending techniques, and with the subsequent development of a new design travelling wave structure, RF feedback systems were no longer required for the construction of compact radiotherapy accelerators (Haimson, J. and Karzmark, C. J., 1963, Brit. J. Radiol., 36, 429).
Apart from clinical applications, compact linear accelerator systems can be effectively employed for industrial radiography and other specialized uses requiring the production and application of megavoltage beams of radiation within a restricted space environment. One such specialized application is the logging of earth formations in which a temperature hardened linear accelerator system, suspended in a borehole, is used to generate a stable high energy electron beam for creating radiation, the interaction effects of which can be analyzed to determine the character and constituents of the earth formations penetrated by the borehole.
Well-logging applications impose severe restrictions on the design of an electron linear accelerator. These restrictions are due to the small transverse dimensions of the pressure housing containing the accelerator equipment (typically 5 inches or less), the low level of available input power (typically less than 1 kW) due to the long borehole logging cable, and the high temperatures encountered during operation (such as 100 to 200.degree. C.). In comparison with prior linear accelerator applications, these design restrictions are unique and should be considered together with the requirement that the borehole accelerator system be operated over distances which may extend to approximately 20,000 feet.
While the design constraints of a borehole linear accelerator system are necessarily severe, the output radiation intensity and energy can be substantially greater than that of the chemical radioactive sources used for existing logging services. Thus, in comparison with a standard cesium well logging radioactive source, a borehole linear accelerator having orders of magnitude greater radiation output, with average and peak photon energies in the megavoltage range, permits measurements of greater statistical precision and permits greater depths of investigation of the geological formation surrounding the borehole. Highly stable and accurately reproducible electron energy characteristics and a low level of electromagnetic interference are additional desirable features which allow simpler and more accurate measurement analysis techniques.
Attempts to overcome the aforementioned restrictions are generally described in U.S. Pat. No. 3,061,725 issued Oct. 30, 1962, to J. Green entitled "Comparison Logging of Geologic Formation Constituents" and U.S. Pat. No. 3,976,879 issued Aug. 24, 1976, to R. Turcotte and entitled "Well Logging Method and Apparatus Using a Continuous Energy Spectrum Proton Source." Because of extreme temperature environments encountered in boreholes, efforts have been made to design cooling and control systems to stabilize the linear accelerator performance. U.S. Pat. No. 4,163,901 issued Aug. 7, 1979, to G. Azam, et al., entitled "Compact Irradiation Apparatus Using a Linear Charged-Particle Accelerator" discloses a particular cooling system inside the housing for the accelerator, particularly for cooling the magnetron RF power source for the linear accelerator disclosed therein. U.S. Pat. No. 4,093,854 issued June 6, 1978, to R. Turcotte, et al., entitled "Well Logging Sonde Including a Linear Particle Accelerator" discloses a standing wave type particle linear accelerator excited by a magnetron oscillator and provided with means to sense variations in the temperature of the accelerator and to adjust the frequency of the magnetron so as to compensate for accelerator resonant frequency variations resulting from temperature induced changes in the dimensions of the accelerator waveguide. This patent also discloses means to sense the variations in the amplitude of the microwave field in the accelerator and to control the frequency of the microwave generator so as to maintain the amplitude of the accelerating field at a reference value representative of the expected maximum amplitude value at resonance. It will be appreciated that the linear accelerators of the Azam et al. and Turcott et al. patents require complex and sensitive measuring devices and controls to provide a useful beam.
Notwithstanding the required controls, these last two mentioned patents do not address the problem of electromagnetic interference (EMI). The geometric constraints of a small diameter cylindrical housing present a unique and potentially serious EMI problem for a borehole linear accelerator because of the necessity to operate low level, relatively sensitive electronics in close proximity to modulator components that are being pulsed at a peak power level of several megawatts. Thus, the interconnecting cables (between power supplies and circuits controlling timing, protection, diagnostic and detection functions) running alongside the pulse forming network (PFN) and the high voltage switch, are susceptible to conductively coupled noise caused by radiated electric and magnetic fields. Coupled noise presents a major problem in the vicinity of a pulsed, fast rise time, high power switch tube. As is well known in modulator art, the HV switch tube enables energy stored in the PFN to be rapidly transferred to a step-up pulse transformer, thereby applying HV video pulses to the cathodes of the RF generator and accelerator gun. For example, in one embodiment of a borehole accelerator modulator, a pulse current of 500 amperes is switched through the primary winding of the pulse transformer for a period of several microseconds.
In addressing the problems of low available power, restricted transverse dimensions, high operating temperature and EMI, applicant has discovered a unique system which provides a very stable form of acceleration and a simple method of using same wherein a megavoltage particle beam is achieved with an accelerator of short length and small diameter and which will produce constant energy particles over a wide temperature range, without EMI interference, without the need to sense the temperature of microwave components, without moving parts, and with restricted input power. This apparatus and method can be achieved without cooling of the RF power source or accelerator structure and without the need for a beam focusing solenoid around the accelerator waveguide.